Recently, developments in semiconductor technology have been concentrated on the fabrication of highly integrated semiconductor devices. In the fabrication of such highly integrated semiconductor devices, it is important to use a photolithography method with a high precision. Furthermore, a linewidth of 0.25 .mu.m or less is required to achieve a high integration of semiconductor devices. However, it is impossible to obtain such a micro linewidth using existing photolithography methods. Currently, such a micro linewidth is obtained via a lithography method like a deep ultraviolet (DUV) process. Generally, photoresists used in the DUV process are chemically-enhanced resists. A variety of chemically-enhanced resists have been developed and are being developed currently. Such chemically-enhanced resists are radiated by DUV rays and then cured in accordance with a baking process carried out after the radiation process, thereby forming desired patterns.
The thermal stability of a baking device, in which a baking process as mentioned above is carried out, serves as an important factor in the fabrication of semiconductor devices. Where the thermal stability of such a baking device is degraded, it is impossible to satisfy a dispersion tolerance in linewidth given for semiconductor devices. This is because the dispersion tolerance in linewidth becomes very small due to a reduction in linewidth to a micro dimension. To this end, most semiconductor manufacturers have made an effort to improve the thermal stability of baking devices. By virtue of such an effort, an improved baking device has been developed in which a heater plate exhibiting a very small temperature dispersion is used as a heating source of the baking device. A baking device including such a heater plate is disclosed in U.S. Pat. No. 4,518,848 issued to Weber.
FIG. 1 schematically illustrates a baking device generally used in the fabrication of semiconductor devices. Such a baking device may be applied to systems manufactured by DNS, Japan bearing trademarks "DNS80A" and "DNS80B" and systems manufactured by TEL, U.S.A. bearing a trademark "MARK". Referring to FIG. 1, the baking device is arranged in the interior of a system housing 308. The baking device comprises a base 300 and a cover 302. A heating plate 304 is mounted on the base 300 to bake a wafer 306 laid thereon. The cover 302 is movable with respect to the base 300 between its open and closed positions. Loading and unloading of the wafer 306 into and out of the baking device are carried out by a robot (not shown). This robot loads the wafer 306 into the baking device and unloads the wafer 306 out of the baking device through a shutter 310 provided on the housing 308.
With such a baking device, however, it is impossible to control a variation in temperature occurring inside of the baking device due to an introduction of ambient cold air into the baking device through the shutter. Such cold air entering the baking device increases a linewidth dispersion beyond the tolerance, as shown in FIG. 2. In FIG. 2, the x-axis indicates the wafer position in the baking device and the y-axis indicates the linewidth dispersion rate of a resist pattern formed on the wafer. FIG. 2 shows that the resist pattern exhibits an instable linewidth dispersion rate at its end portions of the wafer. In order to eliminate such a problem and to maintain a stable linewidth dispersion rate of resist patterns, various proposals as mentioned above have been made. However, conventional methods cannot achieve a stable control over a temperature variation due to ambient air entering the baking device. In particular, the instable linewidth dispersion rate causes more severe problems when resist patterns have a linewidth of 0.25 .mu.m or less.